38 



THE SKELETON 



thick, broad, and somewhat quadrilateral, projects horizontally backward. It is 

 thicker belo\v than above and terminates in a rough posterior edge. The articular 

 processes are thick and strong. The superior articular surface is concave and 

 directed backward and medially; the inferior is convex and looks forward and 

 laterally. The superior pair are more widely separated than the inferior pair and 

 embrace the inferior articular processes of the vertebra above. The posterior 

 margin of each superior articular process is surmounted by the mammillary 

 process or tubercle (metapophysis) which corresponds to the superior tubercle 

 of the transverse process of the last thoracic vertebra. In man the mammillary 

 tubercles are rudimentary, but in some animals they attain large proportions, as 

 in the kangaroo and armadillo. The transverse processes are long, slender, 

 somewhat spatula-shaped, compressed from Ijefore backward, and directed 

 laterally and a little backward. They are longest in the third vertebra and dimin- 

 ish in the fourth, second, and fifth, in this order, to the first, in which they are 

 shortest of all. Their extremities are in series with the lateral tubercles of the 

 transverse processes of the twelfth thoracic vertebra and also with the ribs. 

 With the latter the so-called transverse processes in the lumbar region are homol- 

 ogous, and hence they are sometimes called the costal processes. Occasionally 

 the costal element differentiates and becomes a well-developed lumbar rib. 



Fig. 42. — -A Lumbar Vertebra. 

 (Showing the compound nature of the transverse process. Upper view.) 



Mammillary process 



Accessory process ft 

 or tip of the true ^^ 

 transverse pro- 

 cess 



Costal element 



Costo -transverse 

 foramina 



lifhind the base of each transverse or costal process is a small eminence, directed down- 

 ward, which corresponds with the inferior tubercle of the lower thoracic transverse process, 

 and with the transverse processes of the thoracic vertebra; above, and is named the accessory 

 process (ariapi)i)hysis). The accessory process rei)resents th(; ti]) of the partially suppressed 

 true transverse process of a lumbar vertebra. It is well developed in some of the lower animals, 

 as in the doj? and cat. 



Ivich of th(! five lumbar vertebra; is readily recognized. The ])ody of the first 

 is deeper behind than in front; the body of the second is equal in depth in front 

 and Ix'hind; the bodies of the third, fourth, and fifth are deeper in front than i] 

 beiiind, l)ut the tiiird has long transverse processes and the inferior articular 

 processes are not widely separated. The fourth has shorter transverse processes 

 and the inferior articular processes are placed more widely apart. The fifth 

 lumbar vertebra deviates in some of its features so widely from the other members 

 of the scries that special prominence must now be given them. 



The fifth lumbar vertebra is massive, and the body is much thicker in front 



